I recently installed RH9 and all is going very well. I am able to install rpms and also compile and install source code.

My question...

If I want to install an upgrade of a application, for instance, mozilla or mysql. Do I need to uninstall the old application and then do a new install? Can I just run an upgrade command? If I just run an upgrade command, will the installation be done in a new directory or will it overwrite the old installation.

In Windows, of course, most application upgrades just replace and/or modify files in the old application and the installation is very transparent. I am wondering if this is the same case with linux.

I want to upgrade the applications that came with RH9 to the current versions.

Depends really.

If you are using up2date or getting the RPM's to upgrade from your distro's mirrors, then it should be rather seemless, and the command would be (just an example):
rpm -Uvh filename.rpm

To "Upgrade" an existing install. You can use that for new ones as well in case you aren't sure what's installed and what's not

If you are grabbing source however, you may need to tweak some (alot) of the configurations script (if there is one) to fit the needs of your system, alternatively, you can:
mv program program.old
And then install (if you don't know what I mean, you may wanna stick with RPM's until you get a grasp on source installs ).

Cool

I noticed you used Mozilla as an example if you do want to upgrade the 1.2.something that came with it know that the -U won't work for this I tried forever and it wants to install it separtatly also as I noticed over the last couple of days it's very buggy. prob not worth doing for a little while.